I'll restate my points:
- If your code is well-written with regard to implementing the requirement, then you will not notice much difference once the code is "tuned"
- If your code is badly-written (but still producing the correct output) write the code better to continue producing the correct output
- Hi Akatsukami!
If you there is something which requires real tuning, it is probably not a task for a beginner (except as an exercise, I suppose).
You have to judge the amount of time spent on the tuning/testing against the gain (which you have to fully understand the terms of). Most of the recommendations in the document for which Anuj has provided a link are for CPU-usage reduction. If you task is I/O-bound, it won't help much.
The biggest jump in reduction of CPU (after fixing any bad ways to do something, ie the "badly-written" above) will be to specify the compiler option OPTimise. The biggest jump in reduction of I/O (after fixing any bad ways to do something, ie the "badly-written" above) will likely be in the definition of the datasets/JCL.
Don't go off on your own and "tune" a program from the above document, unless it is an entirely learning-related task.